1. Field of the Invention
The present invention relates generally to wafer processes and equipment, and more particularly, to methods and systems for confirming a process end-point within the multi-step sequence.
2. Description of the Related Art
In the fabrication of semiconductor devices, planarization operations of silicon wafers, which can include polishing, buffing, and cleaning, are often performed. Typically, integrated circuit devices are in the form of multi-level structures on silicon substrate wafers. At the substrate level, transistor devices with diffusion regions are formed. In subsequent levels, interconnect metallization lines are patterned and electrically connected to the transistor devices to define the desired functional device. Patterned conductive layers are insulated from other conductive layers by dielectric materials, such as silicon dioxide. As more metallization levels and associated dielectric layers are formed, the need to planarize the dielectric material increases. Without planarization, fabrication of additional metallization layers becomes substantially more difficult due to the higher variations in the surface topography. In other applications, metallization line patterns are formed into the dielectric material, and then metal planarization operations are performed to remove excess metallization.
Planarizing metallization layers, specifically copper metallization layers is becoming more important as copper has begun to replace aluminum as the metal of choice for metallization processes. One method for achieving semiconductor wafer planarization is the chemical mechanical planarization (CMP) technique. Further applications include planarization of dielectric films deposited prior to the metallization process, such as dielectrics used for shallow trench isolation or for poly-metal insulation.
In general, the CMP process involves holding and rubbing a typically rotating wafer against a moving polishing pad under a controlled pressure and relative speed. CMP systems typically implement a rotating, orbital table or a linear belt in which a surface of a polishing pad is used to polish one side of a wafer. A chemical solution (i.e., slurry) is used to facilitate and enhance the CMP operation. Slurry is most usually introduced onto and distributed over a moving surface of the polishing pad as well as the surface of the semiconductor wafer being buffed, polished through, or otherwise prepared by the CMP process. The distribution of the slurry is generally accomplished by a combination of the movement of the surface of the polishing pad, the movement of the semiconductor wafer and the friction created between the semiconductor wafer and the surface of the polishing pad.
FIG. 1 shows a side view of a conventional linear wafer polishing apparatus 100. The linear wafer polishing apparatus 100 includes a polishing head and carrier 108, which secures and holds a wafer 104 in place during CMP processing. A polishing belt and pad combination 102 (polishing pad) forms a loop around rotating drums 112, and generally moves in a direction 106 at a speed of up to approximately 600 feet per minute, however this speed may vary depending upon the specific CMP operation. As the polishing pad 102 moves, the polishing head 108 rotates (either direction, as chosen by user) and lowers the wafer 104 onto the top surface (i.e., the preparation surface) of the polishing pad 102, loading it with required polishing pressure.
A bearing platen manifold assembly 110 supports the polishing pad 102 during the polishing process. The platen manifold assembly 110 may utilize any type of bearing such as a fluid bearing or an air bearing. The platen manifold assembly 110 is supported and held into place by a platen surround plate 116. Gas pressure from a gas source 114 is input through the platen manifold assembly 110 via several independently controlled of output holes that provide upward force on the polishing belt and pad combination 102 to control the profile of the polishing pad 102.
The polishing pad 102 transports slurry over the wafer surface. Typically the polishing pad 102 has longitudinal grooves 118 (i.e., grooves along the length of the polishing pad 102 in the linear direction 106 the polishing pad 102 travels around the rotating drums 112). A single slurry nozzle, or a multi nozzle dispense bar (with a number of discrete slurry dispense points) 120 dispenses the slurry 122 on the top surface of the polishing pad 102. The position of a slurry nozzle 120 can be adjusted across the width of the top surface of the polishing pad 102. The slurry nozzle 120 is typically aligned in a position relative to the wafer 104 such as center on the wafer 104. However, the position of the slurry nozzle 120 is typically adjusted to somewhat optimize the uniformity of the removal of material from the surface of the wafer 104.
FIG. 2A shows a cross-sectional view of a dielectric layer 202 on a wafer 104 that is undergoing a fabrication process that is common in constructing damascene and dual damascene interconnect metallization lines. The dielectric layer 202 has a diffusion barrier layer 204 deposited over the etch-patterned surface of the dielectric layer 202. The diffusion barrier layer, as is well known, is typically titanium nitride (TiN), tantalum (Ta), tantalum nitride (TaN) or a combination of tantalum nitride (TaN) and tantalum (Ta). Once the diffusion barrier layer 204 has been deposited to the desired thickness, a metallization layer 206 (e.g., copper layer) is formed over the diffusion barrier layer in a way that fills the etched features in the dielectric layer 202. Some excessive diffusion barrier and metallization material is also inevitably deposited over the field areas. In order to remove these overburden materials and to define the desired interconnect metallization lines and associated vias (not shown), a chemical mechanical planarization (CMP) operation is performed on the wafer 104.
There are two basic types of CMP planarization. The first type is topographical planarization, which is used to form a flat surface for a subsequent process. The second CMP type is feature shaping which is forming a flat surface that exposes features (e.g., metal features, contact, lines, vias, etched out regions and features, etc.) in layer recessions.
Determining an accurate end-point to the CMP process (end-point detection) is a very critical aspect of the CMP process. Ending the CMP process too early causes the wafer to be submitted to a re-work process to first determine the amount of additional CMP required to satisfy the desired goal (i.e., fully remove the desired quantity of material). Secondly, the wafer must be actually reworked to fully remove the desired quantity of material. If the CMP process is ended too late can result in too much material being removed resulting in dishing and rounding or even damaging the wafer beyond being salvaged by repair or rework.
Direct endpoint detection methods monitor the wafer surface using acoustic wave velocity, optical reflectance and interference, impedance/conductance, electrochemical potential change due to the introduction of specific chemical agents. U.S. Pat. Nos. 5,399,234 and 5,271,274 disclose methods of endpoint detection for metal using acoustic waves. These patents describe an approach to monitor the acoustic wave velocity propagated through the wafer/slurry to detect the metal endpoint. When there is a transition from one metal layer into another, the acoustic wave velocity changes and this has been used for the detection of endpoint.
Further, U.S. Pat. No. 6,186,865 discloses a method of endpoint detection using a sensor to monitor fluid pressure from a fluid bearing located under the polishing pad. The sensor is used to detect a change in the fluid pressure during polishing, which corresponds to a change in the shear force when polishing transitions from one material layer to the next. Unfortunately, this method is not robust to process changes. Further, the endpoint detected is global, and thus the method cannot detect a local endpoint at a specific point on the wafer surface. Moreover, the method of the U.S. Pat. No. 6,186,865 patent is restricted to a linear polisher, which requires an air bearing.
There have been many proposals to detect the endpoint using the optical reflectance from the wafer surface. These methods can be grouped into two categories: monitoring the reflected optical signal at a single wavelength using a laser source or using a broad band light source covering the full visible range of the electromagnetic spectrum. U.S. Pat. No. 5,433,651 discloses an endpoint detection method using a single wavelength in which an optical signal from a laser source is impinged on the wafer surface and the reflected signal is monitored for endpoint detection. The change in the reflectivity as the polish transfers from one metal to another is used to detect the transition.
Broadband methods typically rely on using information in multiple wavelengths of the electromagnetic spectrum. U.S. Pat. No. 6,106,662 discloses using a spectrometer to acquire an intensity spectrum of reflected light in the visible range of the optical spectrum. Two bands of wavelengths are selected in the spectra that provide good sensitivity to reflectivity change as polish transfers from one metal to another. A detection signal is then defined by computing the ratio of the average intensity in the two bands selected. Significant shifts in the detection signal indicate the transition from one metal to another.
A common problem with each of the above endpoint detection techniques is that some degree of over-polishing is required to ensure that all of the conductive material (e.g., metallization materials 206 such as copper, aluminum or other metal layers and the diffusion barrier layer 204) is removed from over the dielectric layer 202 to prevent inadvertent electrical interconnection between metallization lines and features. For instance, as shown in FIG. 2B, the overburden portion of the copper layer 206 and the diffusion barrier layer 204 have been removed. A side effect of improper endpoint detection or over-polishing is that dishing 208 occurs over the metallization layer that is desired to remain within the dielectric layer 202. The dishing effect essentially removes more metallization material than desired and leaves a dish-like feature 208 over the metallization lines. Dishing is known to impact the performance of the interconnect metallization lines in a negative way, and too much dishing can cause a desired integrated circuit to fail for its intended purpose.
Prior art endpoint detection methods typically, can only approximately predict the actual end point but cannot actually detect the actual endpoint. This approximate prediction increases the likelihood of excess dishing occurring. As a result, the endpoint detection is set to a conservative point resulting in wafers that have still residual areas and portions of metallization layer 206 that must be quantified and removed and in a rework process. Rework processing is extremely costly and inefficient processing. Further, reworked wafers are no longer a standard production product but rather become special products and as such can incorrectly skew quality and reliability results of the production process.
FIG. 3A is a flowchart of a typical CMP process 300 including the rework of the wafers. In operation 302 the wafer 104 is loaded into the conventional linear wafer polishing apparatus 100 (CMP process tool) such as through a loading chamber or port or other entry to the CMP process tool 100.
In operation 304, the wafer 104 is mounted into the polishing head 108 typically by a robot arm. In operation 306 the wafer 104 is planarized as described above until an endpoint is detected in operation 308. The endpoint can be detected by any of a number of prior art methods such as a specific planarization duration or an optical analysis of the wafer through an endpoint detection window in the polishing pad 102 or other methods.
Once the endpoint has been detected, the wafer 104 is removed from the polishing head 108 in operation 310. In operation 312, the wafer is removed from the CMP process tool 100 and transferred to an inspection station where the wafer 104 can be inspected in operation 314. Inspecting the wafer 104 typically includes examining the wafer 104 to determine if any film residues remain. If any film residues remain, the thickness of the film residues must be determined. Once the thickness of any film residue is determined, then a rework process can be developed for the specific rework needs of the wafer 104, in operation 322. The specific rework process is individual to each wafer 104 because the thickness of the film residue on each wafer may be different. In operation 324 the CMP process tool can be programmed to perform the wafer-specific rework process and the wafer 104 can be reworked through the CMP process tool (or a separate CMP process tool) as described above in operations 302-320. If all of the desired material has been removed (i.e., no residual films remain on the surface of the wafer 104), in operation 320, the CMP process is complete and the CMP process ends.
While the prior art process 300 described above purported to detect a CMP endpoint, the actual endpoint for each planarized wafer was not actually detected until after the wafer was removed from the CMP process tool and inspected at an inspection station in operation 314 and no residual films were found to remain on the surface of the wafer 104. Detecting the actual endpoint for each wafer outside of the CMP process tool results in additional handling, a longer and more complex CMP process, and potentially requiring multiple passes by each wafer through the CMP process tool to achieve the desired planarizing results. Further, conventional rework processes results in non-standard product as described above.
In view of the foregoing, there is a need for accurately determining and mapping any residual metal film remaining on the surface of the wafer integral within the CMP process tool.